Handle SCO volume change in multihfpending state.

Use case:
1. Connect one headset.
2. Make MO/MT call.
3. While connecting second headset, change
   call volume on the phone.

Failure:
SCO volume changed is not updated to first headset.

Root cause:
Volume change is not handled in MultHFPending state.

Fix:
Added handling volume change in MultiHFPending state.

Change-Id: I6ac3cb2968b99de194fa7c0114d45f9d81dcb663